Jp. Bourdineaud et al., A Rox1-independent hypoxic pathway in yeast. Antagonistic action of the repressor Ord1 and activator Yap1 for hypoxic expression of the SRP1/TIR1 gene, MOL MICROB, 38(4), 2000, pp. 879-890
Hypoxic SRP1/TIR1 gene expression depends on the absence of haem but is ind
ependent of Rox1-mediated repression. We have found a new hypoxic pathway i
nvolving an antagonistic interaction between the Ixr1/Ord1 repressor and th
e Yap1 factor, a transcriptional activator involved in oxidative stress res
ponse. Here, we show that Ord1 repressed SRP1 gene expression under normoxi
a and hypoxia, whereas Yap1 activated it. Ord1 and Yap1 have been shown to
bind the SRP1 promoter in a region extending from -299 to -156 bp upstream
of the start codon. A typical AP-1 responsive element lying from -247 to -2
40 bp allows Yap1 binding. Internal deletion of sequences within the SRP1 p
romoter were introduced. Two regions were characterized at positions -299/-
251 and -218/-156 that, once removed, resulted in a constitutive expression
of SRP1 in a wild-type strain under normoxic conditions. Deletion of both
these two sequences allowed the bypass of YAP1 requirement in a Delta yap1
strain, whereas these two internal deletions did not yield increased expres
sion in a Delta ord1 strain compared with the full-length promoter. Both a
single Delta ord1 mutant and a doubly disrupted Delta yap1 Delta ord1 strai
n yielded normoxic constitutive SRP1 expression and increased hypoxic SRP1
induction, thereby demonstrating that ord1 is epistatic to yap1. Thus, Yap1
is not directly involved in SRP1 induction by hypoxia, but is necessary to
counteract the Ord1 effect.